Fix special handling of 'zoom vertical' scroll wheel modifier key.

gtkmm2ext/keyboard.cc has a special case to emit a signal on the key-up of
the modifier key used to adjust track heights in conjunction with the
scroll wheel, so that the same track continues to be resized even when
it's shrunk to no longer be under the mouse cursor. However, this code
assumed that the modifier key for this was <Shift>.

Fix it to use the event->state bit corresponding to
ScrollZoomVerticalModifier instead, and rename the relevant functions to
clarify that it's the 'zoom vertical' modifier key they're dealing with.

Partially fixes #5610.